Purification of synthetic methanol



May 20, 1941. J. GlEsEN ETAL- 2,242,453

' PURIFICATION OF SYNTHETIC METHANOL i Filed Aug. 23, 1938 l 552 7,64 vsl v l BY THE/? ATTORNEYS' Patented May 20, 1941 UNITED STATES PAT-ENT iol-FICE PURIFICATION F 'SYNTHETIC METHANL Johann Giesen, Helmut Hansch, and Paul Herrmann Gericke, Leuna, Kreis Merseburg, Germany, assignors, by mesne assignments, to William E. Currie, New York, N. Y.

Application August 23, 1938, Serial No. 226,256

v In Germany September 10, 1937 zclaims.

The present invention relates to an improved process for the purification of Y synthetic meth-v The recovery of a methanol corresponding in its purity to the commercial product from the crude product which-hasbeen obtained in known (o1. 2oz-57) manner -by the catalytic synthesis. from the constituents having an unpleasant odour, in particular unsaturated constituents which greatly impair its usefulness for example as a solvent or an agent protectingv against freezing.

According -to a known process Vit is possible'to remove the said substances and to obtain a product 'corresponding to the pure methanol of commerce by treating the synthetic methanol in the'presence of water with permanganate and zinc chloride. By reason of the strongly corrosive action of zinc chloride, however, the said process can only be carried out in expensive apparatus of corrosion-proof substances,A in particular highly-alloyed steels. It is true that with permanganate alone a vfar--reachingremoval of the impurities is eiected,v but even this is not sufiiciently complete. as hydrogen peroxide, perborates and percarbonates, are also incapable of removing the troublesome odour of the synthetic product. When using persulphates as oxidizing agents, mercaptans are formed which render the odour of the product worse.

We have now found that the said diiiiculties can be overcome by adding chromic acid in small amounts t'o the synthetic methanol to be puried, the process being diagrammatieally depicted in the drawing.

It is especially suitable to use a continuously working distillation column to which at a certain height, for example in the middle part of the column where the methanol to be purified still has a considera-ble watercontent, a dilute Other oxidizing agents; such' aqueous solution of chromic acid' is added in suchan amount that the water flowing away at the lower end of the column is weakly acid to practically neutral.

Since chromic acid has a passivating eiect on iron, the process may be carried out in any iron vessels without the latter being attacked. The oxidizing action'of the chromic acid is restricted to the impurities and injurious further oxidation, as for example to formaldehyde or carbon dioxide, does not take place to a detectable extent.

In this manner, which is very simple' and cheap in operation, there can 4be obtained yfrom the synthetic methanol a methanol which is entirely free from objection as regards its odour.

The following examples will further illustrate how the vpresent invention may be carried out in practice but the invention is not restricted to these examples.

Example 1 For the purification of methanol which has there is introduced at the height of the 20th tray 800 litres per hour of a mixture of methanol and water containing about 40 per cent of l methanol which is preheated to C. At the top of the column about 320 litres of methanol per hour are withdrawn and at the bottom of the column the water is led away at a temperatureof 103 C.

The following table gives the results obtained by simple distillation of methanol without the addition of an oxidizing agent (column II) by distilling the same methanol which has been treated before the distillation with an aqueous potassium permanganate solution (column III) and in column IV the result obtained from'the same methanol when it is treated according to this invention by introducing a 2 per cent aqueous solution of 200 grams of chromic anhydride (CrQz) per hour at the height of the 27th tray where the methanol still has a considerable water content.

For purposes of comparison, the values for pure commercial methanol are given in column I.

Example 2 Into a bell-tray column of- 400 millimetres diameter and having 40 trays there are sprayed in per hour at the height of the 20th [tray 100 litres 5 of a crude methanol obtained from carbon mon- Table Pum methanol Methanol prepared from dimethyl ether I n m rv Spoclc gravity 0.7918-- 0.792. 0.792 0.792. Bromina value.. 0.0. 3.5 1.0 0.8. Autono 0.007%. 0.010 0.(Il30 0.0022. Fecon t- 0.1 m g ll 0.1 mg Il 0.1 mg./l 0.1mg./l. Permanganate stability 2 minutos... l1 minutes.. 4l minutes.

lon Negative A trace.-.-.. Negativa--. Negative. odour Free from obiaction.-- Rnd Bad Very good.

number of cubic centimetres of the said solutionwhich are n tor giving yellow colourstion. A solution of 0.05 gram of KzCnOv in 1000 cubic centimetres of distilled water served as y comparative solution for the yellow colouration.

The permanganate stability referred to in the table was determined as follows:

100 cubic centimetres of the methanol to be Atested are mixed with 1.3 cubic centimetres of a potassium permanganate solution 1:1000. The vessel containing the mixture is kept constant at a temperature of from 17 to 18 C. in a waterbath. The time taken for the red colouration which occurs to disappear is the measure for the 4 permanganate stability.

The results of the investigation given in the table lead to the following conclusions:-

The product obtained by distilling the synthetic methanol without any addition (1I) correspcmds in single points to a rather far-reaching extent with the -pure commercial product (I) so that it is suitable for numerous purposes. inter alla for the preparation of formaldehyde. The bromlne value and permanganate stability are,

however, considerably Aworse than with pure vmethanol and .in particular the bad odour -is objectionable for many purposes. An improvement.4 is obtained by the treatment with te (III) but the methanol thus purified is still greatly inferior to the pure methanol of commerce.

l' The methanol treated according to this invention (.IV) is however, entirely equal to commercial methanol in every respect.'

oxide and hydrogen under a pressure of 200 atmospheres at elevated temperature in the presence of a` zinc oxide-chromium oxide catalyst which has been freed from gaseous constituents and dimethyl ether. At the height of the 30th tray there are introduced at the same time 4 litres per hour of a 2 per cent aqueous solution of chromic acid. The entirely anhydrous methanol leaving the top of the column is distinguished'from that obtained by simple dehydration by a pure odour free from objection.

What we claim is:

1. In the production of pure methanol the steps which comprise distilling crude synthetic methanol obtained by catalytic hydrogenation of car bon oxide under pressure in a distillation column, adding thereto at a height of the said column where the methanol still has aconsiderable water content an aqueous solution of such an amount of chromic acid that the water flowing away at the lowerend of the column is from weakly acid to practically neutral and withdrawing deodorized methanol from the upper end of said column.l y

.2. In the production of pure methanol the steps which comprise distilling crudesynthetic methanol obtained by catalytic hydrogenation of earbon oxides under pressure in a. distillation column. adding thereto in the middle part of the said column such an amount of chromic acid that the water flowing away at the lower end of the col- 50 umn is from weakly acid to practically neutral and withdrawing deodorized methanol from the upper end of said column. 1

JOHANN GIESEN. HELMUT HANISCH. PAUL HERRMANN GERICKE. 

